1. Field of the Invention
This invention relates to a V-6 engine in which a pair of cylinder banks are set at no larger than about 30.degree. to each other.
2. Description of the Prior Art
In V-6 engines, the cylinder banks are generally set at 60.degree. or 90.degree. to each other, and the six cylinders are divided into three pairs, that is, the number 1 and number 4 cylinders are paired, the number 2 and number 5 cylinders are paired, and the number 3 and number 6 cylinders are paired, and the pistons for the cylinders of each pair are designed to reach the top dead center simultaneously. This permits the crankshaft to be designed to be symmetrical about a point and firing to occur at equal intervals, thereby minimizing moment of inertia and torque fluctuation.
Further, it has been proposed to theoretically nullify the linear moment in a V-type engine having an even number of cylinders by setting the relation between the crankpin's relative angle .beta. and the cylinder's included angle (the angle at which the cylinder banks are set to each other) .alpha. to satisfy formula 2.alpha.-.beta.=180.degree.. (See Japanese Unexamined Patent Publication No. 58(1983)-81250, for example.)
In V-6 engines, in order to satisfy the formula while realizing equal interval firing, the cylinder.varies.s included angle .alpha. must be set to 60.degree.. In the case of V-6 engines having a cylinder's included angle .alpha. substantially equal to 30.degree. or smaller than 30.degree., the torque fluctuation and the moment of inertia are increased even if the crankpin positions are determined in accordance with the characteristics described above.
The V-6 engine having the cylinder's included angle of not larger than about 30.degree. is advantageous in that it has both merits that the cylinder head can be formed in one piece and that the overall length of the engine can be shortened, the former being inherent to in-line engines and the latter being inherent to V-type engines, and in that the overall length and the width of the engine can be set to desired values. Accordingly, there is a great demand for a V-6 engine in which the cylinder's included angle is not larger than about 30.degree., and in which the torque fluctuation is reduced and the unbalanced moment of inertia is minimized.